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Recent Advances in Gold(III) Chemistry: Structure, Bonding, Reactivity, and Role in Homogeneous Catalysis
No full textOver the past decade the organometallic chemistry of gold(III) has seen remarkable advances. This includes the synthesis of the first examples of several compound classes that have long been hypothesized as being part of catalytic cycles, such as gold(III) alkene, alkyne, CO and hydride complexes, and important catalysis-relevant reaction steps have at last been demonstrated for gold, like migratory insertion and β-H elimination reactions. Also, reaction pathways that were already known, for example the generation of gold(III) intermediates by oxidative addition and their reductive elimination, are much better understood. A deeper understanding of fundamental organometallic reactivity of gold(III) has revealed unexpected mechanistic avenues, which can open when the barriers for reactions that for other metals would be regarded as "standard" are too high. This review summarizes and evaluates these developments, together with applications of gold(III) in synthesis and catalysis, with emphasis on the mechanistic insight gained in these investigations
Experimental insights into the structure and reactivity of frustrated Lewis pairs
No full textFrustrated Lewis pairs (FLPs), namely the combination of a Lewis acid (LA) and a Lewis base (LB) that cannot fulfil their natural propensity to form stable Lewis adducts, have been recently exploited to activate small molecules, such as H2 and CO2, and have drawn much interest, due to their applicability in metal‐free catalytic reactions. This review focuses mainly on the experimental studies of both the structure and reactivity of FLPs providing a starting body of evidence to elucidate the mechanism of their action. Important insights coming from NMR spectroscopy, X‐ray diffraction and other experimental techniques are highlighted and discussed. In particular, evidence about the tendency of LA and LB to interact in the absence of a substrate are commented on for both inter‐ and intramolecular FLPs, underlining the role played by the structure of LA and LB in determining the level of interaction. Next, the effect of the structure of FLPs on the kinetics and thermodynamics of substrate activation is illustrated, with a particular focus on catalytic hydrogenation reactions
Assessing the orbital contribution in the “spodium bond” by natural orbital for chemical valence–charge displacement analysis
Get PDFThe term "spodium bond"(SpB) has been recently proposed to describe the noncoordinative interaction that can be established between a polarized group 12 metal and a mild Lewis base (LB). Most of the systems showing short metal-donor distances compatible with SpB are characterized by the coexistence of multiple weak interactions, including hydrogen and halogen bonding, making the assessment of real importance of SpB difficult. Here, we show that the relative importance of each contribution can be probed by dissecting the orbital component of the interaction through the extended transition state-natural orbital for chemical valence-charge displacement analysis (ETS-NOCV-CD). The latter gives useful information about relative energies and electrons involved, for model systems ([(thiourea)2MX2]···LB; M = Zn, Cd, and Hg; X = Cl and I; and LB = CH2S, CH2O, CH3CN, and CO) and a variety of structures extracted from experimentally characterized adducts, allowing us to demonstrate the lack of a direct correlation between a favorable metal-base distance and the presence of an orbital contribution for the SpB
Carbon-sulfur bond formation by reductive elimination of gold(iii) thiolates
No full textWhereas the reaction of the gold(iii) pincer complex (C^N^C)AuCl with 1-adamantyl thiol (AdSH) in the presence of base affords (C^N^C)AuSAd, the same reaction in the absence of base leads to formation of aryl thioethers as the products of reductive elimination of the Au-C and Au-S ligands (C^N^C = dianion of 2-6-diphenylpyridine or 2-6-diphenylpyrazine). Although high chemical stability is usually taken as a characteristic of pincer complexes, results show that thiols are capable of cleaving one of the pincer Au-C bonds. This reaction is not simply a function of S-H acidity, since no cleavage takes place with other more acidic X-H compounds, such as carbazole, amides, phenols and malonates. The reductive C-S elimination follows a second-order rate law, -d[1a]/dt = k[1a][AdSH]. Reductive elimination is enabled by displacement of the N-donor by thiol; this provides the conformational flexibility necessary for C-S bond formation to occur. Alternatively, reductive C-S bond formation can be induced by reaction of pre-formed thiolates (C^N^C)AuSR with a strong Brønsted acid, followed by addition of SMe2 as base. On the other hand, treatment of (C^N^C)AuR (R = Me, aryl, alkynyl) with thiols under similar conditions leads to selective C-C rather than C-S bond formation. The reaction of (C^N^C)AuSAd with H+ in the absence of a donor ligand affords the thiolato-bridged complex [{(C^N-CH)Au(μ-SAd)}2]2+ which was crystallographically characterised
Hafnium-pyridylamido organometallics: Unusual reactivity of exceptional catalysts for olefin polymerization
No full textPyridylamido-Hf(IV)-based organometallics have emerged as remarkable catalysts for olefin polymn. having unique properties. Initially reported as the first industrially relevant systems to yield highly isotactic polypropylene with soln.-process technol., they attracted even greater attention for their propensity of undergoing reversible chain transfer with main element alkyls (such as AlR3 and ZnR2), thereby opening the door to novel olefin block copolymers through "chain shuttling" [1]. In this contribution, we report the results of an integrated NMR and DFT study aimed at understanding the reactivity of hafnium pyridylamido complexes with dialkylzinc and trialkylaluminium species [2]. In particular, we show that pyridylamido hafnium cations undergo ligand exchange with ZnR2 or AlR3 thus affording unusual heterobimetallic adducts, in which the cyclometallated naphthyl acts a bridge between hafnium and main element. The results of 1H EXSY NMR expt., carried out to investigate the dynamicity of such adducts, are also reported. Finally, we show how the reactivity of the catalyst towards α-olefins is affected by the presence of ZnR2 or AlEt3
Comparison of PGSE NMR and ESI-MS measurements on methylaluminoxane
No full textPGSE NMR and ESI-MS studies of different grades of hydrolytic methylaluminoxane (MAO) demonstrate a correlation between the average dimensions and anion distribution of MAO. This correlation is revealed through aging and gelation studies of a commercial formulation of MAO. Formulations featuring an anion distribution skewed to higher m/z ratios have been observed to contain significantly higher activator contents as measured by 1H NMR spectroscopy, despite being otherwise very similar. PGSE NMR studies in toluene vs. chlorobenzene media demonstrate that the average hydrodynamic dimensions of MAO are largely unaffected by solvent polarity, although aggregation behavior is somewhat sensitive to solvent polarity. As for catalyst activation and ion-pair speciation, the situation in polar solvents is complicated by the formation of solvated cations (and anions) in chlorobenzene which dramatically lower dimensions. DFT studies of model aluminoxane structures in the size range of MAO featuring a variety of architectures, molar masses and Me3Al contents reveal a linear correlation between Dt, as measured by PGSE NMR, and molar mass using simple relationships and estimates of molecular volume if suitable high molecular weight standards are available. There is reasonable agreement in molar mass with available ESI-MS data, recognizing that MAO is not monodisperse
Pincer complexes of gold: An overview of synthesis, reactivity, photoluminescence, and biological applications
No full textThis chapter discusses the structures, reactivity patterns, and applications of gold(III) pincer complexes, organized according to ligand types: (1) neutral heteroatom pincers type N^N^N; (2) anionic heteroatom pincers N(-)^N^N(-) and E(-)^N^E(-) (E=O, S); (3) neutral C-based pincers C^N^N; (4) monoanionic C-based pincers C(-)^N^N, C^N(-)^C, and N^C(-)^N; and (5) dianionic C-based ligands C(-)^N^C(-) and C(-)^C(-)^N, with emphasis on the more recent literature. The ability of pincer complexes to stabilize previously unknown compounds, such as gold(III) hydrides, alkene, CO, and CO2 complexes is discussed, followed by a discussion on their photophysical properties and the applications as luminescent compounds and a survey of their biological and medicinal activity, especially their role as cytotoxic agents in anticancer studies
Heterolytic bond activation at gold: Evidence for gold(III) H-B, H-Si complexes, H-H and H-C cleavage
Get PDFThe coordinatively unsaturated gold(III) chelate complex [(C^N-CH)Au(C6F5)]+ (1+) reacts with main group hydrides H-BPin and H-SiEt3 in dichloromethane solution at 70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C6H3But)-6-(C6H4But)-pyridine anion; Pin = OCMe2CMe2O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC6F5}2(μ-H)]+ (2+) and (C^N-CH)AuH(C6F5) (5), together with spectroscopically detected [Et3Si-OEt2]+. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature ( 20 °C). H2 activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt2)2]+, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C6F5H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt2 with Hantzsch ester is essentially instantaneous and quantitative at 30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold
Probing the interactions between all components of the catalytic pool for homogeneous olefin polymerisation by diffusion NMR spectroscopy
No full textDiffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp2ZrMe2/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe3 depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C6D6 with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)(2) and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D) MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp2Zr(mu-Me)(2)AlMe2]MeMAO (1) OSIP (outer sphere ion pair) and [Cp2Zr+Me center dot center dot center dot MeMAO(-)] (2) ISIP (inner sphere ion pair) that form upon activation of Cp2ZrMe2. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions
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